From a production of thin monocrystalline silicon layers on a carrier substrate, it is known to first join a silicon wafer to a carrier substrate and to subsequently produce a thin film of typically 5 xcexcm to 50 xcexcm by regrinding and polishing the wafer. These films are, for example, used as solar cells or for the production of electronic circuits on glass, polymers or ceramics.
Alternatively, it is known to allow a single-crystal silicon film having a thickness of 2 to 20 xcexcm to grow epitaxially on a thin film of porous silicon and thereafter to join this grown silicon film to a carrier substrate. Thereupon, the porous silicon film is destroyed or it is detached from the epitaxially grown silicon film resulting in a thin single-crystal silicon film on a carrier substrate. This method is designated the "psgr" process.
In addition, the publication by Gxc3x6sele et al., Appl. Phys. Lett., 70, (11), 1997, 1340 ff. describes how to implant large quantities of hydrogen in a buried layer in a wafer in a silicon wafer so that H2 bubbles are produced by conglomeration of the hydrogen, the H2 bubbles splitting off an overlying thin layer of silicon.
Moreover, conventional thin-film solar cells are based either on the deposition of a photovoltaic, amorphous or polycrystalline layer system on non-adapted carrier substrates such as glass or ceramic. Known techniques for this purpose use, for example, CuInSe, CaTe, a-silicon or polysilicon as the active layer. On the other hand, thin-film solar cells may also be based on deposition of an electronic or photovoltaic system on a lattice-adapted carrier substrate. Known methods for this purpose use, for example, as described above, the silicon epitaxy on silicon or the silicon epitaxy on porous silicon.
Such monocrystalline thin-film solar cells can achieve efficiencies of up to 20%; however, their manufacturing costs are relatively high due to the necessary epitaxial deposition.
One of the objects of the present invention is to provide a method to produce thin, particularly monocrystalline films with a thickness of typically 100 nm to 10 xcexcm in a cost-effective manner on almost any carrier substrates, particularly however, those having high temperature stability. Moreover, these layers should be usable for thin-film solar cells, for example.
The method according to the present invention has the advantage that thin films can be produced on any carrier substrates in a cost-effective manner. In particular, the method according to the present invention is suitable for producing monocrystalline silicon films from conventional wafer material or highly thermostable substrates. Moreover, a plurality of thin films can be produced from one wafer in succession, only a thin sacrificial layer being consumed within the wafer material or the parent body each time so that the presented production method is particularly very cost-effective. In addition, large-area films can be produced with it if necessary.
It is also advantageous that pretextured wafers such as those used, for example, in the production of solar cells can also be used for the method according to the invention.
In addition to the production of thin silicon films, the method according to the invention is also suitable for the production of thin films from a large number of materials that can be made porous in particular, such as germanium or silicon carbide.
In contrast to the "psgr" process, with the use of a silicon wafer, for example, as a starting material for the production of a thin film on a carrier substrate, the film is composed of the original wafer material when the method according to the invention is applied and this possesses a maximum electrical quality. As a result, the method according to the invention is also suitable for the production of very high-quality electronic systems from cost-effective thin film silicon on any, i.e., even flexible carrier substrates, and for the production of high-quality monocrystalline c-silicon thin films on glass such as are required, for example, for thin-film solar cells.
Thus, the thickness of the thin film to be produced on the carrier substrate can be easily adjusted via the depth of the buried sacrificial layer or its distance from the surface of the parent body. The depth at which the sacrificial layer is produced can in turn be adjusted, for example, via the kinetic energy of hydrogen implanted in the parent body, the implanted hydrogen having a sharp stop profile as a function of the energy distribution of the hydrogen in a parent body such as silicon with the ultimate result that the energy and the energy distribution of the hydrogen determines the layer thickness of the thin film to be produced and the thickness of the buried sacrificial layer.
The sacrificial layer can be subsequently detached from the thin film to be produced advantageously by making the sacrificial layer porous by etching or anodization over the entire surface area followed by mechanical or chemical removal of the porous sacrificial layer. If silicon is used as the starting material, the porosity is brought about advantageously by converting the buried sacrificial layer into porous silicon, the structure of which is mechanically unstable and which can be removed easily chemically.
If necessary, this procedure may be followed by a thermal annealing step which improves the quality of the thin film produced by removing radiation damage resulting from the hydrogen implantation.
After the sacrificial layer is removed, the residual body remaining from the parent body can be reutilized to produce additional thin films.
With the aid of the porous silicon technique, the method according to the invention also makes it possible to implement solar cells without cost-intensive epitaxial steps.
Furthermore, a pn junction can be produced advantageously within the thin film to be produced by suitable doping via various methods.
The efficiency of a thin-film solar cell produced with the method according to the invention can be further increased by providing additional layers of defined, but varying porosity and thus varying refractive indices which form a broadband reflection filter on the side of the solar cell facing away from the incidence of light in order, as a result, to reflect the light transmitted through the stack of layers into the active area of the solar cell, i.e., the produced thin film with a pn junction.
It is also possible to increase the efficiency of the produced solar cell by adjusting the porosity of the thin film produced on the carrier substrate to a defined low level. This results in increased light scattering within the produced thin film which, for example, reduces transmission of radiation through the solar cell and consequently results in improved collection of light and a higher production of charge carriers.
Additional advantageous improvements of the efficiency of the solar cell may be obtained by multiple reflections within the solar cell which result from additional surface texturing of individual layers of the solar cell.